High-voltage transformer

ABSTRACT

The invention relates to a high-voltage transformer arrangement which converts to the direct current voltage from a direct current voltage source ( 2 ) into a direct current voltage output signal (V out ), using a current inverter ( 4 ) and a transformer ( 8 ) to an output rectifier ( 12 ), which is connected downstream, and a filter ( 14 ). The transformer ( 8 ) has a transformer core comprising several interruption points in the magnetic path, at which insulator sections are located. The individual core sections are controlled by potential. The separate transformer core sections together with their corresponding windings form a primary system and a secondary system, which is separated from said primary system both spatially and in terms of potential, having a relatively high stray inductance. Said stray inductance may be used in conjunction with other components for the resonant mode of the high-voltage transformer during the switching operations (quasi-resonant mode) and for adjusting an approximately sinusoidal transformer current during the current inverter clock cycle. At the same time, the spatial separation simplifies the insulation of the transformer parts.

[0001] The invention relates to a high-voltage transformer arrangementfor converting a first, relatively low direct voltage supplied by adirect-voltage source to a second, relatively high direct voltage. Ahigh-voltage transformer arrangement of this type can be used, forexample, for charging a capacitor with high capacitance over a shortperiod of time.

[0002] Different designs are known for high-voltage transformerarrangements. The essential elements of a high-voltage transformerarrangement involve a current inverter connected to the direct currentsource and generally formed by an arrangement of power-electronicswitches, a transformer with primary coil and secondary coil on atransformer core, a switch control for triggering the current inverter,an output rectifier that is connected to the secondary coil of thetransformer and a filter connected to the output rectifier.

[0003] For nearly all applications of a high-voltage transformerarrangement of this type, the most compact transformer design possibleis normally desired. The size of the transformer above all is criticalfor the structural volume of the high-voltage transformer arrangement.Essential elements for dimensioning a transformer designed for ahigh-voltage operation are the required insulation between thetransformer components, namely the primary coil or coils, thetransformer core and the secondary coil or coils.

[0004] When viewing a traditional transformer, for example, where avoltage of 40 kv develops against ground on the secondary side, aninsulation designed for 40 kv must be provided between the location ofthe secondary coil where the 40 kV voltage appears and the adjacentlocation, which has ground or mass potential, e.g. the transformer coilthat is connected to ground potential. As a result, narrow limits areset for a compact geometric dimensioning of the transformer.

[0005] It is the object of the invention to provide a high-voltagetransformer arrangement for which the transformer design can besimplified considerably as compared to known high-voltage transformerarrangements.

[0006] This object is solved according to the invention for thishigh-voltage transformer arrangement in that the transformer core isprovided with several interruption points on the magnetic path, in whichinsulation sections are located, and in that the transformer coresections formed by the interruptions, which are preferably controlled bypotential, together with associated coils or coil sections of primaryand secondary coils, form separate primary and secondary systems thatare separated with respect to potential.

[0007] The creation of several “interruption points” in the transformercore creates two insulated sections of the transformer core that arecompletely separate with respect to potential. The term “several” meansthat at least two interruptions are necessary in order to create twosections that are separate with respect to potential.

[0008] The separation of the transformer core into several sections isaccompanied by a complete spatial separation of the transformer into aprimary system and a secondary system. This spatial separation makes iteasier to insulate the area between the primary coil and thecorresponding core sections or the secondary coil and the correspondingcore sections. Separating the transformer according to the inventioninto primary and secondary systems that are insulated against each otherresults in an increase in the stray inductance with simultaneous tappingof the main inductance. This effect, which may not be favorable per se,can be utilized for optimizing the high-voltage transformer arrangementoperation with a corresponding design for the current inverter connectedto the primary system or the output rectifier connected to the secondarysystem.

[0009] The high-voltage transformer arrangement according to theinvention can be used in power supplies with a direct-current voltageintermediate circuit.

[0010] For one special embodiment of the invention, a continuousinsulator is arranged between primary system and secondary system.Starting with this basic measure, the primary systems of a plurality oftransformers can subsequently be combined, in particular inside a jointhousing, which then contains the transformer core sections for theindividual transformers and the associated primary coils. The secondarysystem can be treated in the same way. The primary systems and thesecondary systems can be disposed in separate housings or in a jointhousing. The use of a suitable arrangement or external wiring makes itpossible to control the individual sections of the transformer core orcores with respect to potential, such that the lowest maximum potentialdifferences result between the potentials of respectively one coil andthe potential of the core section assigned to this coil. The insulatordisposed between the core sections must then insulate the differentpotentials of the core sections against each other. For this, individualcore sections of the transformers can be interconnected with specificswitching components (of the current inverter(s) or the outputrectifier(s)), such that the interfering emission from the high-voltagetransformer arrangement is minimized.

[0011] It is particularly advantageous if the energy in the inventivehigh-voltage transformer arrangement of a special embodiment istransmitted without intermediate storage in the transformer and based onthe flux converter principle from the primary system to the secondarysystem. A storage choke coil with thereto-connected output capacitor ispreferably connected downstream of the output rectifier. A storage chokecoil can also be provided in each output branch of the output rectifierto reduce the voltage stress for the single choke coil.

[0012] Comments relating to special, advantageous embodiments of theinvention are provided in the following, wherein these embodiments canbe used separately or in any combination considered useful or obvious tothe person skilled in the art.

[0013] The inventive concept of separating the transformer core intoseveral sections, electrically insulating these sections and theassociated creation of a primary system and a secondary system make itpossible to have a variety of high-voltage transformer arrangementdesigns. One or several primary coils or primary coil sections can beassigned in the known manner to a transformer. In the same way, one orseveral secondary coils or secondary coil sections can also exist. Thehigh-voltage transformer arrangement can be provided with one or severalindividual transformers.

[0014] In principle, the primary coils of several transformers can beparallel connected, wherein these primary coils are then supplied by thesame direct-current source via a joint current inverter. However, theprimary coils of each transformer can also be triggered separately witha separate current inverter. In addition, the primary coils can beconnected in series.

[0015] The secondary coils of several transformers in the secondarysystem can be connected in series, wherein each secondary coil isassigned a separate output rectifier that has a separate output filteror is connected to a joint one. Also possible is a series connection ofthe secondary coils of the high-voltage transformer arrangement, whichis then connected to a joint output rectifier.

[0016] In the following, several circuit variants for connecting theprimary system or systems of a special embodiment of the high-voltagetransformer arrangement are explained.

[0017] The transformer or the interconnected transformers areadvantageously connected on the primary side with one terminal to thecenter tap of a half bridge, consisting of power-electronic switchesthat do not block in return direction, and with the other terminal to acapacitor.

[0018] The transformer of an alternative embodiment is connected on theprimary side with each terminal to a center terminal of a¹ consisting ofpower-electronic switches, which do not block in return direction. Thepower-electronic switches in particular comprise transistors. Thesetransistors either comprise a hybrid integrated or parasitic diode, orthe diode is antiparallel connected as discrete component. It is knownthat power transistors of this type are encumbered with parasiticcapacitances. The above-mentioned increased stray inductance of thetransformer in the high-voltage generator² according to the inventionpreferably can be attuned to the parasitic capacitances of the switchingtransistors, so as to result in a resonant operation for the switchingoperations of the current inverter. If necessary, the capacitance mustbe increased with a discrete capacitor. The switching losses can bereduced considerably with a resonant operation of this type and theelectromagnetic compatibility of the circuit can be improved. Byattuning the stray inductance of the transformer to the parasiticcapacitances of the switching transformer, it is possible to trigger acomplete recharge of parallel capacitances prior to the closing of thetransistor, which must take over the current, thereby minimizing theswitching losses.

[0019] Preferably measures are taken in the secondary system of thetransformer for the high-voltage transformer arrangement according tothe invention to ensure the voltage sustaining capability of the outputrectifier. For that purpose, several diodes are connected in series, ifnecessary, in each branch of the respective output rectifier. Inparticular, respectively one capacitance and one resistance are parallelconnected for each individual diode of the output rectifier. As aresult, the diodes are dynamically and statically balanced with respectto the voltage to be blocked. These capacitors, which areparallel-connected to the diodes in the output rectifier, are preferablyattuned to the stray inductance of the transformer, so as to form aseries resonance circuit with a characteristic frequency that issomewhat lower than the switching frequency of the high-voltagetransformer arrangement predetermined by the switch control.

[0020] As mentioned in the above, the separation according to theinvention of the transformer into a primary system and a secondarysystem, insulated against the primary system, results in an increase inthe stray inductance, which can be used advantageously. At the sametime, the main inductance of the transformer is reduced. If thisreduction in the main inductance is high enough to prevent achieving thedesired operational behavior of the high-voltage transformerarrangement, it is possible according to one preferred embodiment of theinvention to parallel-connect or series-connect the primary coils of atransformer for a high-voltage transformer arrangement containingseveral primary coils, wherein these primary coils can include one orseveral secondary coils. This so-called nesting results in an increasein the main inductance of the transformer.

[0021] In accordance with the above measure, one special embodiment ofthe high-voltage transformer arrangement has several secondary coilsthat are connected parallel or in series, wherein one or several primarycoils are also included. For this, the coils of the secondary system canalso be conducted out either individually or with a joint tap, so thatseveral secondary voltages—even completely insulated secondaryvoltages—are made available.

[0022] As a result of the inventive separation of the transformer in ahigh-voltage transformer arrangement with an insulator or individualinsulator sections inserted between primary system and secondary system,it is possible to meet relevant regulations for insulation design withreduced technical expenditure.

[0023] Insulating systems that are known per se can be expanded by theinsulator sections between primary system and secondary system in thehigh-voltage transformer arrangement according to the invention. Inparticular, the thickness, expansion and edge shape of the interruptionsin the transformer core can be designed in dependence on the requiredvoltage sustaining capability between primary-side and secondary-sidecore sections of the transformer and the transformer coils.

[0024] The insulator sections in particular can consist of severaldifferent materials. The material combinations are selected so as toensure on the one hand the required insulating characteristics and, onthe other hand, take over special mechanical functions.

[0025] Specifically, the idea is to design the insulator sections of thetransformer so as to form components of a housing that encloses theprimary system and/or the secondary system. Thus, the switchingcomponents can be sealed hermetically against each other if necessary.In that housing, the insulator separating the primary system from thesecondary system can form the container wall. The primary-side and thesecondary-side regions of the housing, which are separated by a wall,can be filled separately with gas, a liquid, a powder or the like toimprove, for example, the cooling and/or insulation of the switchingcomponents in these regions of the housing.

[0026] It is particularly advantageous if the switch is controlled witha phase-locked loop. The first input of this loop is supplied with acurrent detector signal, derived from the chronological course of theprimary transformer current, while the second input is supplied with theoutput signal from the phase-locked loop is supplied via a delayelement. For this, the power-electronic switches are triggered with theoutput signal and the complementary output signal from the phase-lockedloop.

[0027] The phase-locked loop consists in a manner known per se of aphase detector or phase comparator, a low-pass filter and avoltage-controlled oscillator (VCO) that is connected downstream of thelow-pass filter. The current-detector signal is detected by a currentsensor in the primary coil circuit for the transformer.

[0028] The phase-locked loop ensures a continuous adjustment of thetrigger signal generated by the control circuit for triggering thepower-electronic switch. As a result of the delay element between outputand input of the phase-locked loop, the signals for triggering thepower-electronic switch, which are tapped directly at the output of thevoltage-controlled oscillator for the phase-locked loop, ensure thatthese switches are always switched just before the zero passage of theswitch current. The current inverter thus is ensured to operateinductively and a switching can be realized without loss. To achieve aswitching without loss, the delay is adjusted such that the energystored in the stray inductance at the moment of interrupting the currentinverter is high enough to completely recharge the parasiticcapacitances of the power-electronic switch. Following this, the currentruns freely over the antiparallel diode of the newly closed transistorand the voltage over this transistor is clamped to the diode fluxvoltage. As a result, the respective switch can be closed withoutlosses. The switches are subsequently triggered with complementarysignals, meaning signals offset by 180°, wherein short-circuits in thecurrent inverter are avoided as a result of the lag time between theedges of the trigger-signals for the power-electronic switches, so thatthe desired complete recharging of the transistor capacitances ispossible.

[0029] The adjustment of the upper and lower edge frequency of thevoltage-controlled oscillator in the phase-locked loop preferably occursin such a way that with a suitable lower edge frequency, the triangularcurrent course present during the excitation phase does not exceed amaximum current value. The upper edge frequency is selected such thatthe clocking frequency of the current inverter is always higher than thecharacteristic frequency of the current inverter, which is predeterminedby the stray inductance of the transformer and the capacitors that areparallel connected to the output rectifier.

[0030] As previously explained in the above, several transformers in thehigh-voltage transformer arrangement according to the invention canpreferably be provided with separate current inverters. Modules can thusbe formed, which respectively consist of a current inverter, atransformer primary system, a transformer secondary system, an outputrectifier and an output filter, preferably comprising a smoothing chokeand a capacitor.

[0031] Especially when using several modules of this type, thecomponents for the secondary parts can be dimensioned for smallervoltages. The individual modules can respectively be operated and testedseparately and individual modules can be replaced.

[0032] According to one special embodiment of the invention, which isparticularly suitable for the modular transformer design, several (N)current inverters are cyclically clocked with the same frequency, butwith offset phase angle relative to each other. In particular, a phaseangle of 360°/N is provided between the individual reversing operationsof the current inverters.

[0033] Several identical modules of this type are preferably operated inthe stationary mode and with the same clocking frequency, wherein thephase position of the trigger signals for the current inverters of theindividual modules is fixed on the basis of the operation of a singlemodule. The primary transformer current course of a single moduledetermines the operation of the remaining transformers (master-slaveprinciple).

[0034] Through a cyclical clocking of several current inverters withsignals having the same frequency, but offset phase angles, the rippling(ripple) of the current for the intermediate-circuit capacitor and thusalso the thermal stress of the capacitors in the intermediate circuitcan be reduced. The cyclical switching through of the individual currentinverter operations can be realized with the aid of the above-mentionedmaster-slave principle by connecting a logic circuit to the output of aphase-locked loop, preferably via a frequency divider. By using asuitable logic circuit of the type of a ring counter, trigger signalsthat are staggered in time are generated for the individual (N) currentinverters. With each current inverter, the output signals and thecomplementary output signals of the phase-locked loop are used astrigger signals, that is to say during half a period duration of a cycle(output signal) or a complementary half period of a cycle (complementaryoutput signal). Attention must be paid to the fact that the signalsbelonging to a transformer or a current inverter do not overlap.

[0035] The above-mentioned frequency divider, in particular, operateswith a division ratio TV=2*N (N represents the number of transformers orcurrent inverters that are interconnected to form a module), wherein adecimal ring counter is connected downstream of the frequency divider.The decimal ring counter counts from zero to 2*N−1 and is provided witha separate output for each state.

[0036] For example, with N=5 transformers and current inverters, tenoutputs of the decimal ring counter are connected in pairs to a total offive RS flip-flops, wherein each flip-flop delivers at its output or itscomplementary output a trigger signal or drive signal or a complementarytrigger signal or drive signal for an associated current inverter.

[0037] To ensure that all transformer sections have the desiredpotential, cores made from core materials with poor electricalconductivity, e.g. ferrite, must be provided with a conducting layerthat should not, however, encompass the magnetic flux.

[0038] This conductive layer usefully is designed so as to permit a goodinsulation relative to the associated coil (primary coil or secondarycoil). The conducting layers functioning as potential control electrodesor, with core materials having high conductivity, the individual coresections are electrically coupled to each other via impedances andconnected to the desired electrical reference potentials. This measurepermits an easy adjustment of the desired potential ratios, meaning thelowest possible maximum potential difference is achieved between thepotentials for one coil and the potential of the associated transformercore section.

[0039] The conductive layer on the transformer core sections can be usedadvantageously for adjusting the desired potential ratios without usingadditional elements for the potential control. To do so, severaltransformers of a high-voltage transformer arrangement are arranged insuch a way that the desired potential ratios adjust themselves throughthe capacitive effect of the potential control electrodes.

[0040] Exemplary embodiments of the invention are explained in thefollowing with further details and the aid of drawings, which show in:

[0041]FIG. 1 A section through a transformer for a high-voltagetransformer arrangement according to the invention;

[0042]FIG. 2 A block diagram for an embodiment of a high-voltagetransformer arrangement according to the invention;

[0043]FIG. 3 A circuit diagram for a possible configuration of aninterconnection of N transformers for a high-voltage transformerarrangement according to the invention;

[0044]FIG. 4 A circuit diagram for an alternative embodiment of aninterconnection of N transformers for a high-voltage transformerarrangement according to the invention;

[0045]FIG. 5 A circuit diagram of another alternative interconnection oftwo transformers for a high-voltage transformer arrangement according tothe invention, provided with a voltage divider for the potential controland a choke coil in each output branch of the output rectifier;

[0046]FIG. 6 A circuit diagram of a high-voltage transformerarrangement, comprising a transformer and the components that determinethe characteristic frequency of the power circuit;

[0047]FIG. 7 A circuit diagram of a high-voltage transformer arrangementwith phase-locked loop for triggering the current inverter of thehigh-voltage transformer arrangement;

[0048]FIG. 8 A circuit diagram of a high-voltage transformer arrangementwith N modules and a master-slave trigger logic for the separate currentinverters of the individual N modules;

[0049]FIG. 9 A detailed circuit diagram of the logic for generatingtrigger signals, which is shown in the center of FIG. 8;

[0050]FIG. 10 A pulse diagram for explaining the operation of thecircuit according to FIG. 9.

[0051] Reference is made first of all to FIG. 2, which shows ahigh-voltage transformer arrangement according to a preferred embodimentof the invention in the form of a block diagram.

[0052] The high-voltage transformer arrangement generally given thereference 1 in FIG. 2 comprises a direct-current voltage intermediatecircuit 2 as direct-current source, which is supplied on the input sidedirectly with a direct-voltage signal or via a rectifier with analternating voltage signal V_(in) and which transmits a direct-voltagesignal to a current inverter 4. This current inverter is shownsymbolically as switch, but contains a plurality of power-electronicswitches in practical operations. A control circuit 6 that is configuredas switch control triggers the current inverter 4.

[0053] The current inverter 4 alternately switches the output signalfrom the direct-voltage intermediate circuit to a primary coil of atransformer 8. An output rectifier 12 is connected to the secondary sideof the transformer 8, which is followed by a downstream-connected outputfilter in the form of a low-pass filter 14. The increased direct voltageV_(out) may be tapped at the output of this low-pass filter.

[0054] The transformer 8 is shown in further detail in FIG. 1. Thecontinuous insulator 10 in the transformer 8 in particular divides thetransformer into a primary system P and a secondary system S that iselectrically insulated from the primary system.

[0055] As shown in FIG. 1, the insulator 10 takes the form of a housing,consisting of the outside housing 10 a of a mechanically stablematerial, e.g. a hard plastic, which surrounds the total transformer 8,and two separate inside parts 10 b of the housing, made of a highlyinsulating material. The two inside housing parts 10 b of insulator 10define an imaginary separation line X for dividing the total transformer8 into the primary system P and the secondary system S, to which voltageis supplied via an input terminal pair E or from which voltage is tappedvia an output terminal pair A.

[0056] The primary system P comprises a transformer core section 20 pand a primary coil 22.

[0057] The secondary system S comprises a secondary coil 23, consistingof two secondary coil sections 23 a and 23 b, on a secondary transformercore section 20 s (a single continuous secondary coil 23 is possible aswell).

[0058] An important element of the transformer 8 shown in FIG. 1 is theinsulator 10, which divides the transformer spatially and electricallyinto the primary system P and the secondary system S. The special designof insulator 10 is not limited to the above-described embodiment. Atotally different housing can also be provided or separate, individualinsulator sections between the interruptions (21 a, 21 b and 21 c), inthe transformer core can also be provided for forming the sections 20 pand 20 s.

[0059] As a result of the electrical and spatial separation oftransformer 8 with the aid of insulator 10, the stray inductance Lσ oftransformer 8 is increased and its main inductance L (Lp, Ls) isdecreased.

[0060] The advantages, explained in further detail in the above, of aspatial and electrical separation or insulation of primary system P andsecondary system S are made clearer with the following detaileddescription of exemplary embodiments. As a result of the spatialseparation, the insulation between the individual coils or coil sectionsand the transformer core sections can be realized more easily than withstandard transformers. A favorable potential control is achieved at theindividual transformer locations through a suitable wiring, which isexplained further in the following. This potential control brings themaximum possible potential differences to values, which permit asimplified insulation between coil and transformer core.

[0061] First of all, we want to look at the high-voltage transformerarrangement shown in FIG. 3, wherein the current inverter 4 and thecontrol circuit 6 on the primary side were omitted. Possible embodimentsfor the current inverter supplying the primary voltage V_(p) follow fromthe description below.

[0062] According to FIG. 3, N separate transformers with a design asshown in FIG. 1 are provided. The individual transformers are given thereferences T1, T2, . . . TN. The primary coils for transformers T1 to TNare parallel connected and respectively receive the primary alternatingvoltage V_(p).

[0063] The secondary coils for the individual transformers T1 to TN areindividually connected to a Graetz bridge, wherein each Graetz bridge 24₁, 24 ₂ . . . 24 _(N) transmits a rectified voltage via an output choke26 ₁ . . . 26 _(N) to an associated output capacitor Cs1, Cs2, . . .CsN. The individual capacitor voltages Vs1, Vs2, . . . VsN add up to atotal output voltage Vs.

[0064]FIG. 4 shows an alternative embodiment, for which the individualprimary coils of N transformers T1, T2, . . . TN are respectivelyprovided with a separate current inverter 4 ₁, 4 ₂ . . . 4 _(N).

[0065] On the secondary side, the individual secondary coils are seriesconnected. The sum of the secondary voltages is transmitted from aGraetz bridge 24, via a choke coil 26, to a capacitor C_(s).

[0066]FIG. 5 shows a portion of a high-voltage transformer arrangementwith two transformers T1 and T2, for which the primary coils areparallel connected and the secondary coils are respectively connected toa Graetz bridge. The rectified voltage of each Graetz bridge 24 ₁, 24 ₂is supplied via storage choke coils 26 ₁₁, 26 ₁₂, 26 ₂₁, 26 ₂₂ to a pairof capacitors Cs11, Cs12 or Cs21, Cs22 with the same values, wherein aresistor R11, R12 or R21, R22 with the same value is parallel-connectedto each capacitor.

[0067] The voltage divider, comprising the capacitors Cs11 to Cs22 andthe resistors R11 to R22, provides respectively one fourth of the outputvoltage from tap to tap. The potentials are connected to correspondingcore sections on the secondary side of the divided transformers.

[0068] For example, the voltage at each of the secondary coils for thetwo transistors T1 and T2 is 20 kV in this case. Relative to ground ormass, a potential of 40 kV therefore is present on the upper capacitorCs11 surface, a potential of 30 kV on the upper capacitor Cs12 surface,a potential of 20 kV on the upper capacitor Cs21 surface and a potentialof 10 kV between the capacitors Cs21 and Cs22.

[0069] The secondary-side core section of transformer 1 is connected to30 kV while the secondary-side core section of transformer 2 isconnected to 10 kV. As a result, the insulation between the coil and theassociated core section for each of the transformers must only bedesigned for a maximum of 10 kV. In contrast, the insulation for thetransformer T1 without complete division between primary and secondaryside and complete ground connection of the core, as realized in FIG. 5only for the primary side, would have to withstand 40 kV.

[0070]FIG. 6 shows the high-voltage transformer arrangement 1,schematically shown in FIG. 2, with further details. The currentinverter 4 that is connected to the direct-voltage source 2 in this casecomprises a total of four transistor switches, which can consist ofindividual transistors, parallel-connected transistors or transistormodules. These are shown symbolically and are given the references SW1,SW2, SW3 and SW4. A parasitic capacitance is associated with eachtransistor switch SW1 . . . SW4, which is shown in FIG. 6 on the lefttop with CSW1, parallel to the switch SW1. A diode DSW1 is furthermoreconnected parallel to the capacitance CSW1. In addition to the parasiticcapacitance, a discrete capacitance can also be present, which isparallel connected to the parasitic capacitance. As a simplification,only one capacitor CSW1 is shown for the switch SW1 in FIG. 6. The sameconditions as for switch SW1 also exist for the remaining switches SW2,SW3 and SW4.

[0071] The transformer 8 has a main inductance Lp on the primary sideand L_(s1) and L_(s2) on the secondary side, as well as a strayinductance Lσ, wherein Lσ in turn has primary and secondary components.

[0072] On the secondary side, the two inductances formed with thesecondary coil connections 22 a and 22 b are provided with a center tap,wherein this transformer represents an additional embodiment.

[0073] The secondary system S contains the secondary coils 22 a and 22 bwith the associated inductances Ls1 or Ls2, the output rectifier 12 andthe low-pass filter, connected thereto, with choke coil 26 and outputcapacitor Cd. The output rectifier in each branch of the secondary coilsection in this case comprises a series-connection of diodes D11, D12 .. . D1N in the upper branch or D21, D22 . . . D2N in the lower branch.

[0074] Respectively one capacitor and one resistor C_(D11) . . . or R₁₁. . . is parallel connected to each diode.

[0075] The series-connection of the diodes serves to increase thevoltage sustaining capability in the output rectifier 12. Theparallel-connection consisting of capacitance and resistance for eachdiode serves the purpose of making the diodes symmetric with respect tothe voltage to be blocked. Furthermore, the capacitors together with thestray inductance of the transformer form a series resonant circuit,which causes a sinusoidal transformer current starting with a specificoutput voltage.

[0076] The control circuit 6 in FIG. 6 is shown schematically as ablock. Details of the control circuit 6 are explained further in thetext below, in connection with FIGS. 7 and 8. The embodiment of thecontrol circuit 6 itself is possible for the person skilled in the artbecause of the person's general knowledge. However, the aspectsexplained in the following must be taken into consideration for thespecial course of operation.

[0077] In the secondary system S shown in FIG. 6, current flows via thediode branch D11 . . . D1N or D21 . . . D2N into the output capacitorCs, meaning alternating. That is to say, respectively one diode branchis conducting and the other one blocks, and vice versa. By means of theblocked diode branch with parallel-connected capacitors, a nearlysinusoidal negative voltage builds up, wherein a displacement currentflows through the capacitors. Once this displacement current through thecapacitors C_(D11) . . . C_(D1n) or C_(D21) . . . C_(D2n) has reached anintensity to match the intensity of the current flowing in the outputcapacitor Cs, the current flow through the previously conducting diodesof the other diode branch then reaches zero. A sinusoidal, negativevoltage then builds up above this diode branch, which is now blocking.The voltage above the first blocked diode branch then reaches positivevalues and is clamped to the flux voltage of this branch, wherein thecurrent simultaneously commutates from the capacitor branch to theparallel and previously blocked diode branch.

[0078] The sum of diode current and capacitor current corresponds to thecourse of the primary side transformer current—taking into considerationthe transformation ratio of the transformer—and thus also insection-wise to the switch current.

[0079] If the high-voltage transformer arrangement according to FIG. 6is operated with constant output voltage, a suitable dimensioning of thecomponents will achieve that sinusoidal transformer currents are alwaysflowing in the stationary mode of operation.

[0080] The energy stored in the stray inductance Lσ brings about in thecurrent inverter 4 for the high-voltage transformer arrangement shown inFIG. 6 that the energy stored in the stray inductance during eachoperational step causes a charging and discharging of the capacitancesCSW1 . . . of the transistor switches SW1 to SW4.

[0081] A complete recharging operation occurs if, during the switchingoff operation, the energy stored in the stray inductance Lσ is higherthan the energy stored in the capacitances. That is to say, thecapacitance belonging to the transistor to be opened is charged and thecapacitance belonging to the transistor to be closed is discharged.Following this discharging operation, the voltage present at thistransistor is clamped to the value of the forward voltage for theantiparallel-switched diode. The switching on subsequently occur at avoltage of nearly zero and almost without loss.

[0082] The current-carrying transistors must be opened sufficientlyprior to the zero passage of the current, or the stray inductance mustbe increased, so that the energy stored in the stray inductance Lσ ishigh enough. Under some circumstances, the use of an additionalinductance should also be considered. Also following from this is therequirement that the characteristic frequency of the resonant circuitformed by the capacitances of the output rectifiers 12 and the strayinductance must be lower than the clocking frequency for triggering theswitches SW1-SW4 in the current inverter 4.

[0083] With a suitable triggering of current inverter 4 and matchingdimensions for the components in the primary system P and the secondarysystem S, the circuit shown in FIG. 6 can be operated in the quasiresonant operation that ensures a switching without losses.

[0084]FIG. 7 shows an embodiment of a high-voltage transformerarrangement consisting of one transformer. The main inductance Lp, Lsand the stray inductance Lσ of this transformer are shown. Also shownare the two transistor switches SW1 and SW2 with associated driver(driver 1, driver 2) of the current inverter 4. The secondary systemconsists of one Graetz bridge 24, a choke coil 26 and an outputcapacitor Cs.

[0085] The primary coil is connected with one terminal to the center tapof a half bridge formed by the two switches SW1 and SW2 and with theother terminal to the center tap for two capacitors Cp1 and Cp2. Theswitches SW1 and SW2 are transistor switches that do not block inreverse direction, such as the ones shown in FIG. 6 for the currentinverter 4.

[0086] The control circuit 6 for triggering the current inverter 4(shown here in a form that is modified from the one shown in FIG. 6) isshown on the lower right side in FIG. 7. The control circuit 6 comprisesa phase-locked loop 30 with downstream-connected delay element 32. Thephase-locked loop 30 comprises a phase detector (PD) or phase comparator34, which is known per se, to which a low-pass filter 36 (RC element)and a voltage-controlled oscillator (VCO) 38 are connected.

[0087] A current sensor DET detects the current flowing through theprimary coil, which is converted to a voltage with the aid of a resistor42. The voltage reaches the first (upper) input of the phase detector34. The output signal from the delay element 32 is transmitted to itssecond input. The output signal from the VCO 38 is supplied to thedriver 2 for activating the switch SW2. A negator 40 inverts the signalwithout overlap in the edge region. The inverted signal then travels tothe driver 1 to control the switch SW1. The two signals, which aresupplied via the driver to the switches SW1 and SW2 do not overlap intime, thus preventing a bridge short-circuit.

[0088] The circuit shown in FIG. 7 permits an exact detection of thezero passages of the current with the current sensor DET. Thephase-locked loop 30 causes the switches SW1 and SW2 to open and closeat the correct times, wherein the delay of the delay element 32 isadjusted such that the current inverter 4 switches before the zeropassage of the current in the transformer primary coil. A switchingfrequency adjusts in the process, which is higher than thecharacteristic frequency of the resonant circuit formed by thetransformer stray inductance and the capacitors of the output rectifier.

[0089] A suitable dimensioning of the VCO with lower and upper edgefrequency makes it possible to limit the current in the starting phaseto a maximum value or achieve that with increasing characteristicfrequency, the control frequency or the clocking frequency of thecurrent inverter is still higher than the characteristic frequency.

[0090] Since the switching of current inverter 4 is controlled with thesignal from the output of the VCO 38, the reversal always occurs beforethe zero passage. Thus, the current inverter always operates inductivelyand the switching can be realized without loss, provided the delay inthe delay element 32 is adjusted as explained in the above.

[0091]FIG. 8 shows a high-voltage transformer arrangement, comprisingseveral (N) modules, wherein each module has a similar design as thecircuit shown in FIG. 7. However, only a single control unit 6 isprovided for all modules 1 . . . N. Each module has a transformer T of asimilar type as the one shown in FIG. 1 and is provided with a separatecurrent inverter and a separate secondary system. The output capacitorsCs are connected in series, so that the resulting total output voltagecorresponds to the sum of the individual capacitor voltages Vs1 . . .VsN.

[0092] The upper part of the circuit shown in FIG. 8 resembles thecircuit according to FIG. 7, except that a frequency divider 39 in thephase-locked loop is connected downstream of the VCO 38. The VCOsupplies the frequency divider 39 with a relatively high outputfrequency, which is higher by the divisional factor TF than thecharacteristic frequency and is cophasal. Thus, the lock-in frequency ofthe phase-locked loop 30 is relatively low as compared to the outputfrequency from the VCO 38.

[0093] The relatively high output frequency from the VCO 38 is fed to alogic circuit 50. The logic circuit 50, comprising a ring counter andshown in further detail in FIG. 9, supplies trigger signals orcomplementary trigger signals to N pairs of outputs for the drivers 1.1,1.2 or 2.1, 2.2 . . . N.1, N.2 of the N transformers 1 to N. Thus, thecurrent inverters 4 of all transformers T1 to T5 are switched with thesame frequency, but cyclically displaced in phase,, such that thereversal occurs with a phase difference of 360°/N.

[0094] The high-voltage transformer arrangement shown in FIG. 8 istherefore controlled by the control circuit 6 on the basis of themaster-slave principle.

[0095]FIG. 9 shows the logic circuit 50 in further detail. The outputsignal from the frequency divider 39 is transmitted to an input of adecimal ring counter 52, which has 2*N outputs, here numbered 0, 1, 2, .. . N−2 . . . N, N+1, . . . , 2*N−1.

[0096] From the output pairs (0, N), (1, N+1) . . . lines lead to theset input or the reset input of respectively one RS flip-flop 54 ₁ . . .54 _(N), the output or complementary output of which supplies thetrigger signals for the drivers in modules 1 to N.

[0097] With the aid of a pulse diagram, FIG. 10 shows the driver signalsfor the current inverters in transformers T1 to T5. The complementarydriver signals are not shown, but their course can be seen in thecircuit shown in FIG. 9. Over a total of ten output clocking cycles ofthe VCO, the switch pair SW1, SW2 of each module 1−5 (=N) is switched onand off once.

Patent claims
 1. A high-voltage transformer arrangement, comprising adirect voltage source (2); with at least one current inverter withpower-electronic switches (4); at least one transformer (8; T; T1 . . .TN) having i) at least one primary coil (22), ii) at least one secondarycoil (23), and iii) one transformer core (20 p, 20 s); a switch control(6) for triggering the current inverter (4), such that the directvoltage source (2) is alternately connected on the primary side to theat least one transformer (8; T; T1 to TN); at least one outputrectifier, which is connected to the at least one secondary coil, and atleast one filter (14) that is connected to the output rectifier (12);characterized in that the transformer core (20 p, 20 s) has severalinterruption points (21 a, 21 b, 21 c) on the magnetic path whereinsulation sections are located, and that the sections (20 p, 20 s)formed by the interruptions in the transformer core together with theassociated coils (22, 23) or coil sections (23 a, 23 b) of primary andsecondary coils, form primary and secondary systems (P, S) that areseparated with respect to potential.
 2. A high-voltage transformerarrangement according to claim 1, characterized in that a continuousinsulator (10) is arranged between the primary and the secondary system(P, S).
 3. A high-voltage transformer arrangement according to claim 1or 2, characterized in that the direct-voltage source (2) is formed by adirect-current voltage intermediate circuit, connected via a rectifierto an alternating voltage source.
 4. A high-voltage transformerarrangement according to one of the claims 1 to 3, characterized in thatthe energy is transmitted without buffer storage in the transformer fromthe primary to the secondary system, based on the flux converterprinciple, and that at least one storage choke coil (26) with adjoiningoutput capacitor (Cs1, Cs2 . . . CsN) is assigned to the outputrectifier.
 5. A high-voltage transformer arrangement according to one ofthe claims 1 to 4, comprising several secondary coils, characterized inthat the secondary coils have a center tap and the output rectifier (12)is designed as center-tap connection (FIG. 6).
 6. A high-voltagetransformer arrangement according to one of the claims 1 to 4,comprising several secondary coils, characterized in that the secondarycoils are not provided with a center tap and the output rectifier isdesigned as a Graetz bridge (24).
 7. A high-voltage transformerarrangement according to one of the claims 1 to 6, characterized in thatthe transformer or the interconnected transformers are connected on theprimary side with one terminal to the center tap of a half bridge (SW1,SW2), consisting of power-electronic switches that do not block inreverse direction, and with the other terminal to a capacitor (Cp1,Cp2).
 8. A high-voltage transformer arrangement according to one of theclaims 1 to 6, characterized in that the transformer or theinterconnected transformers are connected on the primary side with eachterminal to the center tap of respectively one half bridge, consistingof power-electronic switches (SW1-SW4), which do not block in reversedirection (FIG. 6).
 9. A high-voltage transformer arrangement accordingto one of the claims 1 to 8, characterized in that during the switchingoperation, the stray inductance (Lσ) of the transformer is used togetherwith the parasitic capacitances (CSW1 . . . ) of the power-electronicswitch (SW1 to SW4) and, if necessary, capacitances that areadditionally parallel connected to these switches for a resonantoperation.
 10. A high-voltage transformer arrangement according to oneof the claims 1 to 9, characterized in that in order to increase thevoltage sustaining capability in each branch of each output rectifier(12), several diodes (D11, D12 . . . D1n; D21, D22 . . . D2n) areconnected in series.
 11. A high-voltage transformer arrangementaccording to one of the claims 1 to 10, characterized in that acapacitance (C_(D11), C_(D12) . . . ) and a resistance (R11, R12 . . . )with the same values are parallel-connected to each output rectifierdiode in order to balance the diodes with respect to the voltage to beblocked.
 12. A high-voltage transformer arrangement according to one ofthe claims 1 to 11, characterized in that the stray inductance of thetransformer (8) together with the capacitances of the output rectifier(12) form a series resonant circuit, which is attuned such that itscharacteristic frequency is somewhat lower than the clocking frequencyof the high-voltage transformer arrangement.
 13. A high-voltagetransformer arrangement according to one of the claims 1 to 12,comprising several primary coils within a transformer, characterized inthat the primary coils are connected parallel or in series and alsoinclude one or several secondary coils.
 14. A high-voltage transformerarrangement according to one of the claims 1 to 13, comprising severalsecondary coils within a transformer, characterized in that thesecondary coils are connected parallel or in series, wherein severalsecondary coils enclose one or several primary coils.
 15. A high-voltagetransformer arrangement according to one of the claims 1 to 14,characterized in that the insulator sections are composed of one orseveral different materials (10 a, 10 b).
 16. A high-voltage transformerarrangement according to one of the claims 1 to 15, characterized inthat the insulator sections represent components of a housing thatencloses the primary system (P) and/or the secondary system (S).
 17. Ahigh-voltage transformer arrangement according to one of the claims 15or 16, characterized in that the regions belonging to the primary systemand the secondary system are separated by a wall and that the individualregions are filled, independent of each other, with an insulating gas,an insulating liquid or an insulating solid material, in particular apowder.
 18. A high-voltage transformer arrangement according to one ofthe claims 1 to 17, characterized in that the control circuit (6) isprovided with a phase-locked loop (30), the first input of which issupplied with a current detector signal derived from the chronologicalcourse of the primary transformer current while the second input ofwhich is supplied with the output signal of the phase-locked loop via adelay element (39) and that the power-electronic switches (SW1, SW2) aretriggered with the output signal and the complementary output signal ofthe phase-locked loop (30).
 19. A high-voltage transformer arrangementaccording to one of the claims 1 to 18, characterized in that severaltransformers are connected parallel on the primary side and in series onthe secondary side.
 20. A high-voltage transformer arrangement accordingto one of the claims 1 to 19, characterized in that several transformersare parallel-connected on the primary side and that on the secondaryside of each transformer a separate rectifier with respectivelyassociated smoothing choke and respectively associated output capacitoris provided, wherein the output capacitors are connected in series. 21.A high-voltage transformer arrangement according to one of the claims 1to 18, characterized in that each transformer is provided on the primaryside with a separate current inverter (4), which is connected to a jointvoltage source (2), so that individual modules are formed, whichrespectively consist of a current inverter, a transformer, an outputrectifier, a smoothing choke and an output capacitor.
 22. A high-voltagetransformer arrangement according to claim 21, characterized in that theseveral (N) current inverters are cyclically clocked with the samefrequency, but offset by a predetermined phase angle.
 23. A high-voltagetransformer arrangement according to claim 22, for which the phaseangles for N current inverters are offset by 360°/N relative to eachother.
 24. A high-voltage transformer arrangement according to claim 22or 23, characterized in that the clocking frequency and the phaseposition of the trigger signals for all modules (M1 . . . MN) arederived from the course of the primary transformer current of one module(M1).
 25. A high-voltage transformer arrangement according to claim 24,characterized in that the control circuit is provided with aphase-locked loop (30) with voltage-controlled oscillator (VCO)(38) onthe output side, wherein the phase-locked loop (30) has adownstream-connected delay element and a frequency divider is positionedbetween the phase-locked loop and the delay element, and wherein phaseoffset trigger signals for the several (N) current inverters are derivedfrom the output signal of the phase-locked loop (30).
 26. A high-voltagetransformer arrangement according to claim 25, characterized in that adecimal ring counter (52) is connected downstream of the frequencydivider (39), which counts from zero to 2*N−1 and has a separate outputfor each state, wherein pairs of the decimal ring counter outputs arerespectively conducted to an RS flip-flop belonging to respectively onemodule (M1 . . . MN), the outputs of which supply the trigger signalsfor the individual modules (M1 . . . MN).
 27. A high-voltage transformerarrangement according to one of the claims 1 to 26, characterized inthat the individual sections of the transformer core (20 b, 20 s) areelectrically connected to reference potentials and via impedances.
 28. Ahigh-voltage transformer arrangement according to one of the claims 1 to27, characterized in that the transformer core sections are coated witha conducting layer (19), which functions in the manner of potentialcontrol electrodes and is connected with reference potential and viaimpedances to other potential control electrodes, wherein the conductinglayer does not encompass the magnetic flux.
 29. A high-voltagetransformer arrangement according to claim 28, characterized in thatseveral transformers are arranged in such a way that the desiredpotential ratios adjust as a result of the capacitive effect of thepotential control electrodes among each other.
 30. A transformer for ahigh-voltage transformer arrangement according to one of the claims 1 to29.